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88                            METALLURGY OP IRON AND STEEL.
be eliminated, this broad formula being profoundly influenced by the skill of the puddler and the purity of the reagents. The chemical history of the puddling process is shown by Table III-A.
SEC. Hid.—Effect of the' temperature upon puddling.—The temperature of the furnace has an important bearing on the character of the product, particularly when much carbon is present. Experiments by Stead* show that in the refining process, which corresponds to the first part of the puddling process, the elimination of phosphorus was inversely as the temperature, ranging from 46 per cent, in hot charges to 91 per cent, with cold working, in each case about 96 per cent, of the silicon and 30 to 40 per cent, of the carbon being oxidized. For many years the phenomenon was explained by supposing that phosphorus would not unite with oxygen at high temperatures^ and this was deemed to be proven by the fact that phosphorus was not burned in the acid Bessemer converter. It is now known that the reduction of phosphorus by high heat in the puddling furnace is due to the simple fact that carbon has a greater affinity for oxygen as the temperature rises, so that it reduces the phosphate of iron and returns the phosphorus to thd metal.
It is the practice at most works to remove part of the slag while . the metal is high in carbon, the product being called "boilings," while the slag which is left in the furnace at the end of the opera*-tion and which is sometimes tapped from the bottom is called "tappings." This last cinder is often allowed to remain, or, if tapped, is charged with the next heat to furnish a rich slag in the early part of the process, since the fettling of iron ore is so infusible that it cannot furnish a cinder until a high temperature is attained. The removal of the "boilings" during the operation hastens the work, gives less cutting of the bottom, and renders the "balling" easier. It also aids. dephosphorization, for during the first part of the operation the charge is at a low temperature, and the slag carries a higher percentage of phosphorus than it would- retain if it were kept in the furnace and exposed to a high temperature and the reducing action of carbon. By tapping during the first part of the boil, the greater part of the silica and phosphorus is removed and there is an opportunity to make a new slag richer in iron.and of
* Journal I. and 8. I., Vol. II, 1877, p. 372,